As a cemented carbide widely used for cutting of metal, a WC—Co alloy which is composed of a hard phase wherein tungsten carbide WC is a main component, and a binder phase of iron-group metals, such as cobalt), or an alloy wherein a carbide, a nitride, a carbonitride, etc. of metals of group 4a, 5a, or 6a in the periodic-table were further added to the WC—Co is known.
Generally, as a method of manufacturing this cemented carbide, a method comprising the steps of: grinding, mixing and molding a raw material powder which constitutes the above cemented carbide, and sintering at 1350–1600° C. for about 1 to 3 hours, is known.
These cemented carbide is mainly applied to cutting of a cast iron, a carbon steel, etc. as a cutting tool. Recently, as for a cemented carbide, application to cutting of a hardly machinable material represented by stainless steel is also considered.
However, since such a cutting difficult material has characters such as generation of work hardening, high affinity with tool material and low thermal conductivity, many problems has generated in the field of cutting. That is, a cemented carbide which has toughness and hardness is needed for processing of a stainless steel.
When cutting of the hardly machinable material, such as a stainless steel, is carried out with a cutting tool made from K-grade cemented carbide which is composed of WC—Co system cemented carbide specified to JIS B 4053 (1996) which is comparatively few amounts of Co, or a cutting tool made from P-grade cemented carbide which has B1 type (cubic type) solid solution of single composition, wear of a cutting tool progresses rapidly, or a fracture whose welding is considered to be a cause is generated, a processing surface state of cutting material gets worse. As a result, it becomes a tool life for a short time, and good cutting can not be performed.
Moreover, a damage to primary notch parts with a cutting force received from a processing surface which carried out work hardening is intense, and it results in a tool life immediately, and comes to acquire good cutting characteristics.
Furthermore, a conventional cemented carbide contains an iron (Fe) and a chromium (Cr) as an impurity. When such a cemented carbide is used as a cutting tool, Fe and Cr combine with a large amount of an iron (Fe) and chromium (Cr) which are contained in a workpiece of which a temperature was raised during cutting. As a result, welding or agglutination of the workpiece to the cutting tool surface is carried out, and action parts (piece edge etc.) are unusually worn out, or a cutting force is increased, whereby it becomes easy to generate damage on a cutting tool surface.
Moreover, there was a problem that a finished-surface coarseness of a surface to be cut deteriorates by an unevenness of a welding thing or an agglutination thing.
An iron (Fe) and a chromium (Cr) in are contained in a primary raw material as an unescapable impurity, or are contained in the cemented carbide during a manufacturing process, and cannot be perfectly removed on industry. Moreover, a content of iron (Fe) and chromium (Cr) which are contained during a manufacturing process is uncontrollable, since it is changeable in connection with change of process and surface states of a grinder or the like.
Moreover, since iron has high affinity with carbon, if a content of iron (Fe) in a surface of the cemented carbide is large, carbon and iron (Fe) combine preferentially, in coating a hard coat by vapor phase synthetic methods, such as CVD and PVD. Accordingly, it becomes easy to generate embrittlement phases, such as η phase, to an interface of the cemented carbide and the hard coat, and an adherence strength of a hard coat falls. Consequently, the hard coat is exfoliated and destroyed, or a life falls in using as cutting tool or slide member.
In order to improve a wear resistance, a method of coating a hard coating of higher hardness on an alloy surface is known. In order to relax an impact to the hard coating, the method of forming the so-called β-free layer wherein a content of B-1 type solid solution is reduced, to a surface area to which a hard coating of the cemented carbide is formed is known.
Furthermore, Japanese Unexamined Patent Publication No. 6–93473 discloses that a content of Zr existing in a depth region of 1–50 μm from a base material surface to insides is disappeared or decreased, when using Ti and Zr as a B-1 type solid solution (without using Nb).
However, it is known that when surfaces of these cemented carbides are oxidized and deteriorated with a heat at the time of cutting and oxygen in environment, its hardness and toughness fall. For this reason, even when a hard coating is coated on an alloy surface, an alloy surface maybe exposed to an oxidizing atmosphere by existence of a defective portion in a hard coating. Especially, if a β-free layer is formed in an alloy surface (that is, P1suf/Pin<0.9, and q1 suf/qin<0.9, each sign of which is defined as an after-mentioned), it will be easy to generate oxidization and deterioration of an alloy surface.
On the other hand, when not forming a β-free layer directly under a hard coating (P1suf=P2suf=pin, q1suf=q2suf=qin, each sign of which is defined as an after-mentioned), the shock resistance and fracture resistance of the hard coating will fall.
Furthermore, like a coating cemented carbide disclosed in Japanese Unexamined Patent Publication No. 6-93473, when there are few contents of Zr in a surface region of a base material (q1suf/qin<0.9, each sign of which is defined as an after-mentioned), plastic deformation resistance worsens and wear resistance falls.